This invention relates to hydraulic systems for controlling a plurality of actuators such as hydraulic cylinders which are found, for example, in aerial work platforms. In such a system, it is not uncommon to provide a pilot operated directional valve for each actuator which is controlled by a manually operated controller through a pilot hydraulic circuit. The directional valve functions to supply hydraulic fluid to the actuator to control the speed and direction of operation of the actuator. In addition, the directional valve for each actuator controls the flow of hydraulic fluid out of the actuator.
In aerial work platforms and the like, the manually operated controller is on the elevated platform and long pilot lines extend from the source of pilot pressure to the manually operated controller and from the controller to the directional valve. Each function of the valve includes a manually operated controller and respective pilot lines to and from the directional valves. In addition, a common tank line is provided from all of the controllers. Such long lines result in a sluggish response that makes it difficult to precisely position the aerial work platform. The long lines also add weight and are costly. In some instances, dual pilot lines are provided where a second controller is provided at the base of the aerial work platform. The weight of the pilot lines often necessitates the addition of counter weights to the aerial work platform which adds to the difficulty of moving the platform along the terrain.
In such systems where pilot pressure is provided to the directional valve from a remote location, the long pilot pressure hydraulic lines especially at cold temperatures result in a large pilot pressure drop which prevents adequate system response to the hydraulic signal initiated by the controller.
It has heretofore been suggested that the directional valves be controlled by electrohydraulic valves on the directional valve with electric wires extending to a manually operated controller on the aerial work platform. Such systems may include solenoid operated pressure reducing valves that provide a pilot pressure to the directional valve. However, it has been found that in the environment in which such systems are used, as in the case of an aerial work platform, the system is more susceptible to malfunction. Furthermore, the owners of such vehicles are usually lessors and find great difficulty in obtaining skilled personnel for maintaining mechnical, hydraulic and electronic systems. The high frequency repair and difficulty in obtaining qualified personnel for maintenance have resulted in the demand for systems which are exclusively hydraulic for various purposes such as aerial work platforms with the aforementioned problems and difficulties of inadequate response, weight and cost.
Such problems also exist in the hydraulic systems shown in U.S. Pat. Nos. 4,201,052 and 4,480,527, having a common assignee with the present application. The hydraulic systems shown therein are intended to accurately control the position and speed of operation of the actuators. In such systems, the directional valves comprise pilot operated meter-in valves and separate pilot operated meter-out valves. A pilot controller supplies pilot pressure selectively to the meter-in valve to apply pressure to one of the lines of the actuator and to open the meter-out valve of the other line to the actuator. Provision is made for sensing the maximum load pressure in one of a series of valve systems controlling a plurality of actuators and applying the higher pressure to the load sensing pump system. In addition, load drop check valves are provided preventing return flow to the meter-in valve when it is in neutral. Inherent leakage in the meter-in valve can adversely affect the hydraulic signal especially in cold temperatures by providing substantial back pressure.
Among the objectives of the present invention are to provide a system which results in rapid response to a hydraulic signal from a controller for all operating conditions; which overcomes the problems of long pilot lines especially in cold weather; which permits the use of smaller pilot lines and smaller hydraulic controllers thereby reducing the weight and cost; and which in one form provides for smooth starting and stopping of a load and accurate positioning of the load, as in high inertia loads such as swing drives.
In accordance with the invention, a hydraulic control system comprising a hydraulic actuator having opposed openings adapted to alternately function as inlets and outlets for moving the element of the actuator in opposite directions, a pump system for supplying fluid, and a directional valve provided to which the fluid from the pump is supplied for controlling flow to and from the actuator. A pair of lines extends from the directional valve to the respective openings of the actuator. A controller alternately supplies a first fluid pilot pressure to pressure reducing valves associated with the directional valve for reducing the pressure from the pump system or any other source and supplying a second reduced pilot pressure to the directional valve for controlling the flow to and from the actuator. Preferably, the directional valve comprises a meter-in valve and a meter-out valve associated with each line to the actuator for controlling flow out of the actuator. Each meter-in valve and meter-out valve is operated by the second pilot pressure from the pressure reducing valve. In a modified form, novel means are provided for achieving smooth starting and stopping and positioning of a load, as in high inertia load such as swing drives.